Magnetic plates of the type referred to above are, for example, illustrated in EP-A-0 211 329, EP-A-0 254 939, EP-A-0 311 865, EP-A-0 345 554, WO-A-09/034425, WO-A-09/130721, WO-A-09/130722, and WO-A-10/125592.
Magnetic plates of this type constitute permanent electromagnetic systems in which the electrical energy is used only for a few seconds in the MAG (activation) and DEMAG (deactivation) steps. The magnetic plate has a distribution of permanent-magnet modules arranged according to an orderly array and having their North/South poles oriented alternately to generate a high concentration of force through a grid of multiple magnetic stitches that enable a horizontal and flattened circulation of the magnetic flux. The anchoring force generated enables a workpiece set on the magnetic plate to be withheld irrespective of the direction of the forces to which the workpiece is subject during machining and also when small thicknesses are involved. The force of attraction is uniformly distributed over the entire surface of contact with the workpiece, without any compression and deformation of the workpiece, unlike what occurs with traditional mechanical clamping devices. The magnetic plate leaves the workpiece free except for just the side of contact with the plate, which favours a better exploitation of the machining unit.
A device as set forth in the preamble of claim 1 is known from document U.S. Pat. No. 4,777,463 A.
The object of the present invention is to improve the flexibility of use of magnetic devices of the type referred to above, with simple and low-cost means, in particular guaranteeing the possibility of use also with workpieces of a complex conformation, and without jeopardizing the precision of positioning of the workpiece.
With a view to achieving said purpose, the subject of the invention is a magnetic device for gripping and clamping a workpiece in a machining unit or on a machining line, which has the characteristics of claim 1.